1. Field of the Invention
The present invention relates to an excitation control circuit for exciting (or energizing) a coil of a solenoid, and in particular, relates to a technique for performing excitation while suppressing rapid attenuation of excitation current.
2. Description of the Related Art
Recently, in various technical fields such as a field for controlling mechanical vibration, solenoids are effectively used. In order to control the attraction of the solenoid (i.e., the operation when the coil of the solenoid is excited), the coil is excited by chopping or the like during the attraction, while in order to control the release of the solenoid (i.e., the operation when the coil is not excited), the coil is not energized during the release.
Here, counter-electromotive voltage (or force), which may damage the excitation control circuit, is produced when energization of the coil is terminated. Therefore, in a known technique, the counter-electromotive voltage is suitably absorbed using a flywheel diode or a Zener diode, and the absorbed voltage is returned to the coil. In addition, Japanese Unexamined Patent Application, First Publication No. Hei 6-132116 discloses an excitation control circuit having a combination of a flywheel diode and a Zener diode, so as to obtain desired attenuation characteristics of the coil in consideration of the counter-electromotive voltage.
FIG. 4 shows the general structure of the excitation control circuit having a combination of a flywheel diode and a Zener diode. In this conventional circuit, the switch 32 is turned on when the attracting operation of the solenoid is performed, and the coil 30 is excited by chopping via the driving circuit 20. The counter-electromotive force produced during chopping is returned via the diode 31 to the coil 30. When the solenoid is released, the switch 32 is turned off, and the counter-electromotive force produced due to the switching-off operation is absorbed by the Zener diode 12 and the diode 11, so that the absorbed force is returned to the coil 30.
More specifically, the coil 30 is a constituent of a solenoid (not shown), and the output of the driving circuit 20 is connected to the positive electrode (see the reference symbol P in FIG. 4) of the coil 30, and the negative electrode of the coil 30 is grounded. In addition, the counter-electromotive force absorbing circuit 10, which consists of the diode 11 and the Zener diode 12, is connected to the coil 30 in parallel, and the counter-electromotive force absorbing circuit 10 and the driving circuit 20 function as the excitation control circuit.
In this structure, when the driving circuit 20 stops excitation of the coil 30, counter-electromotive force is produced at the coil 30 so as to compensate the attenuation of the excitation current which flows through the coil 30. When the voltage of the counter-electromotive force exceeds the breakdown voltage of the Zener diode 12, return current IA flows through the counter-electromotive force absorbing circuit 10, so that the counter-electromotive force of the coil 30 is limited to a fixed level. That is, the Zener diode 12 absorbs a portion of the counter-electromotive force of the coil 30, thereby relieving the damage on the driving circuit 20.
However, in this conventional technique, the counter-electromotive force produced at the coil 30 during the release operation of the solenoid is absorbed by the Zener diode 12; thus, the current returning to the coil 30 is excessively attenuated. Therefore, in order to secure required attenuation characteristics in the release of the solenoid, the coil 30 must be strongly energized by the driving circuit 20. As a result, power consumption is increased.
In addition, the burden imposed on the Zener diode 12 is increased according to the counter-electromotive force produced by the coil 30, so that the heat generated by the Zener diode 12 is also greatly increased, which may affect the operations of other electronic parts or devices.